This invention relates to an ink jet printing system, and more particularly to an improved method and means for maintaining a supply of ink for the printing system.
In the operation of a synchronous ink jet printing device, the ink jet nozzle is supplied with fluid ink, which is pressurized to typically between 2 and 5 atmospheres absolute pressure. Of the ink drops directed at a target surface, those which are not to be used in printing are captured and removed. This unused ink is then returned to the ink supply and recirculated through the system. The ink recirculation heretofore has been achieved by using a pump to supply pressurized ink to the nozzle and draw ink from a vented container. The return ink would be deposited in this vented container. The problems which arise with this method, are those of a low mechanical reliability of the pumps and valves, as well as the difficulty in obtaining chemical compatibility of different inks which are used. The pumps also produce cyclic pressure variations, pulses, which must be minimized by means of fluid pressure regulation. In order to avoid a deterioration in print quality a stable stream of constant velocity droplets must be provided. For this, an ink jet nozzle must be supplied with a nonvarying ink pressure. Thus, the problems introduced by the pumps can adversely affect printing.
Another widely used method of fluid pressurization and recirculation, is to supply a nozzle with fluid ink which is forced out of a pressure vessel by a volume of a pressure regulated compressed gas. The returning unused ink is temporarily stored in another container. When the vessel from which ink is supplied to the nozzle is empty of ink (or nearly so), the ink recaptured from the ink drop stream is introduced into the pressure vessel, from the return ink container, by either automatic means, or manually while the pressure vessel is vented to the atmosphere. This method circumvents the problems associated with pump systems, however, the time which this type of system can operate continuously is limited by ink pressure vessel capacity and the number of nozzles being supplied.